The etiology of tumors is very complex, often one carcinogenic factor can induce multiple tumors, and one tumor may have multiple etiologies. In general, the etiology of most tumors has not been fully understood so far. It is now generally accepted that most tumors are caused by the interaction of environmental factors with the genetic material of cells. The term “environmental factors” refers to such things as cigarettes, dietary components, environmental pollutants, drugs, radiation, and infectious agents (i.e., chemical, biological, and physical factors). However, the fact that some individuals develop tumors while others live beyond a normal lifespan when exposed to the same specific environment suggests that individual factors such as genetic characteristics, age, gender, immune and nutritional status play an important role in the development of tumors. Since the 20th century, studies on epidemiology, high incidence areas and occupational cancers have provided a lot of reliable clues and bases for finding and determining the causes of tumors. (a) Smoking: There have been many studies on the etiology of lung cancer. The relationship between smoking and lung cancer has been proven by a large number of facts. Smoking is a recognized cause of lung cancer, but the proportion of smokers suffering from lung cancer is less than 20%. Smoking can cause not only lung cancer but also oral cancer, hypopharyngeal cancer, esophageal cancer, gastric cancer, bladder cancer, lymphoma, and cardiovascular disease. (ii) Radiation and ultraviolet light: Exposure to ionizing radiation from natural or industrial, medical and other sources can cause various cancers, including leukemia, breast and thyroid cancers. Bone, the hematopoietic system, and the lungs are organs that are sensitive to radiation. Sunlight is a major source of ultraviolet radiation, and long-term UV exposure can cause skin cancer, especially in highly exposed Caucasian populations. Very low frequency electromagnetic fields may also be related to cancer, but it is not conclusive. (iii) Chemical carcinogens: Many compounds are carcinogenic. For example, benzo(a)pyrene contained in cigarettes is strongly carcinogenic and can cause skin cancer and lung cancer. Aflatoxin produced by aflatoxin contaminated food (moldy cereals) may cause liver cancer. Arsenic can cause skin cancer, lung cancer and liver cancer. Currently recognized chemical carcinogens are also asbestos, chromium, nickel, coal tar, mustard gas, mineral oil, dichloromethyl ether, and so on. It is currently believed that the most important chemical carcinogens for the total human cancer risk are the many carcinogenic components of cigarettes. Cigarette filters are inherently carcinogenic if they are produced informally. It has been reported that 85% of male lung cancer patients and 46% of female lung cancers are caused by smoking and passive smoking is even more harmful. Other chemical carcinogens are mainly combustion and organic synthesis products, certain food components, microbial contamination products or substances produced by food preparation processes. In addition, certain physiological and pathological processes in the human body itself such as inflammation, oxidative stress reactions, nutritional and hormonal imbalances, and repeated tissue damage can also produce carcinogenic chemicals such as oxygen free radicals. It is estimated that chemical carcinogenic factors dominate among human cancers caused by environmental factors. (iv) Microbial infection: Although most tumors are not transmissible, it is clear that certain RNA viruses such as human T-cell leukemia virus-1 (HTLV-1) and HTLV-2 viruses can cause leukemia and lymphoma; certain DNA viruses such as hepatitis B virus (HBV) and hepatitis C virus (HCV), EBV, and high-risk human papillomavirus (HPV) can cause liver cancer, hepatitis B and hepatitis C cancer, respectively. can cause liver cancer, Burkitt’s lymphoma, nasopharyngeal cancer, Hodgkin’s lymphoma, and cervical cancer, respectively. More recent data also suggest that Helicobacter pylori (H. pylori, Hp) is also carcinogenic and associated with the development of gastric lymphoma. At least eight viruses have now been shown to be associated with a number of tumors in humans, although the degree of certainty of their association varies. Other biologic factors that cause cancer include some bacteria and parasites. Information from 1995 estimates that biological factors cause 18% of all human tumors. (v) Chronic diseases: Many sources indicate that cancer is prone to develop on the basis of chronic scarring. For example, chronic inflammation of gastric mucosa caused by Helicobacter pylori infection is the basis for the occurrence of gastric cancer. Chronic ulcers of the skin that do not heal for a long time may become cancerous. In northwest China, skin cancer caused by hot bed burn scar is often called “kang cancer”, and there are many colorectal cancers in areas with high incidence of schistosomiasis, which may also be the result of chronic infection. (vi) Nutritional factors: Nutrition and cancer are also closely related. It is estimated that 1/3 of all human cancers are caused by nutritional factors. It is undoubtedly necessary and useful to further determine the role of these factors in the long and complex development of human cancers. Vitamin A and its analogs (commonly known as vincristine) are associated with epithelial differentiation. The results of large prospective population-based observations carried out in New York and Chicago, USA, also illustrate that the intake of natural vitamin A b-carotene in food is negatively associated with the development of several types of cancer over a decade later, most notably lung cancer. Another striking aspect is the relationship between colorectal cancer and fatty diets. Colorectal cancer has been the second most common tumor in Singapore for more than 30 years after the founding of the country, which is especially worthy of our reference. It has been proved that excessive calories and obesity can lead to an increased incidence of breast cancer, colorectal cancer and pancreatic cancer. (vii) Immunosuppression: The incidence of cancer in patients who require long-term application of immunosuppression for organ transplantation is significantly higher than that of the general population. AIDS patients are prone to multiple angiosarcomas (Kaposi’s sarcoma) and lymphomas. The potential hazards should be carefully weighed when long-term immunosuppression is required for various diseases. (viii) Genetic factors: Most human tumors are caused by environmental factors. However, the same exposure to specific carcinogens causes some individuals to develop but not others; in addition, some tumors have significant familial aggregation. These facts suggest that the occurrence of tumors is also related to individual genetic factors. It is currently believed that environmental factors are the initiating factors for tumorigenesis, while individual genetic characteristics determine susceptibility to tumors.